why oil which is rare than water has a refractive index higher than that of water ?

Question

anonymous · Answer

The refractive index of a medium is the medium's response to a traveling electromagnetic field in it. In the case you are probably interested, this electromagnetic field has the frequency of oscillation of visible light. 

The refractive index is, in its most basic form, defined as the relation between the velocity of this light in the medium and in vacuum:

$$n = c / v$$

It is related to the electric properties of the particles which make this medium, and now related, as far as I am aware, to how rare a certain substance is.